Of the aromatic C8 isomers, including the three xylene isomers and ethylbenzene, paraxylene is of particularly high value since paraxylene is useful in the manufacture of synthetic fibers and resins. Refinery and chemical plant streams containing the aromatic C8 isomers typically contain, at thermodynamic equilibrium, only about 22-24 wt % paraxylene, based on the weight of the xylene isomers in the stream. Separation of paraxylene from the other C8 isomers requires superfractionation and/or multistage refrigeration steps and/or adsorptive separation, all of which are energy intensive. There is a need to provide processes for producing paraxylene in more efficient ways, such as in higher selectivity than can be obtained from refinery and chemical plant streams.
One known method for producing paraxylene selectively involves the alkylation of toluene and/or benzene with methanol and/or dimethylether (DME) over a solid acid catalyst. Selectivities to paraxylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material, preferably a medium-pore zeolite and particularly ZSM-5, having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). See U.S. Pat. Nos. 6,423,879 and 6,504,072.
WO 99/38823 reported a reactive distillation process comprising the contact of toluene with a methylating agent in the presence of a zeolite in a reaction/distillation column produces, as a side product, DME, which can be recycled (with unreacted methanol) to extinction in the process. The process operates at no greater than 320° C.
It has recently been discovered that the alkylation of benzene and/or toluene with methanol can also result in the production of a variety of oxygenates, in addition to DME, but also other oxygenate by-products. See for instance U.S. patent application Ser. No. 13/487,651. According to the invention described in Ser. No. 13/487,651, the concentration of phenolic impurities in a xylene stream produced by alkylation of benzene and/or toluene with methanol can be reduced to trace levels, e.g., below 0.1 ppmw, by one or more washing treatments with an aqueous solution of a base. The resultant treated xylene stream, if necessary after water washing to remove any phenate-containing solution, can then be recycled to the xylene splitter to generate additional para-xylene or can be used as a solvent. This discovery was surprising, as phenol is not present in reformate streams, which is the traditional source of xylene streams.
It has also recently been discovered that xylenes produced by alkylating toluene and/or benzene with an alkylating agent comprising methanol and/or DME over a solid acid catalyst contain small quantities of styrene, which, if not removed, could cause operability problems for downstream paraxylene recovery processes, or even further, in processes using paraxylene, such as the production of terephthalic acid, and derivatives thereof, including polyester fibers, films, and the like. See co-filed U.S. patent application Ser. No. 13/875,373.
Several characteristics of the xylene produced by alkylation of benzene and/or toluene make styrene removal challenging. The desired product, paraxylene, is present at higher-than-equilibrium concentration. The material used to remove phenol must therefore show minimal xylenes isomerization activity. The catalyst must also minimize formation of benzene, which also can have detrimental effects on downstream processing. Furthermore, as discussed above, the product may also contain styrene as well as other olefinic compounds that may enter the alkylation reaction system via the feedstream of toluene, such as catalytic reforming units, which are a source of toluene for the aforementioned alkylation reaction. These and other problems make the treatment of the product stream from the alkylation of benzene and/or toluene in the presence of an acid catalyst difficult.
It has long been known that certain substances have selective adsorption characteristics useful in resolving mixtures into component parts. See, for instance, U.S. Pat. No. 4,453,029, which teaches the use of certain zeolites in selective separation of certain mono- and para-disubstituted benzene ring compounds and mono-substituted pyridine ring compounds in admixture with aromatic compounds selected from the group consisting of ortho-disubstituted, meta-disubstituted and more highly substituted benzene ring compounds and poly-substituted pyridine ring compounds.
Other relevant prior art includes U.S. Pat. No. 6,555,611, teaching an absorbent for aromatic hydroxyl compound comprising composite metal oxide solid solution particles or hydrotalcite-like composite metal hydroxide particles; JP 56039025A, teaching removal of phenols from organic matter by contact with a polyvinyl pyridine resin; CN 1253937A, teaching using silica gel to remove phenol-like compounds from styrene monomers; U.S. Pat. No. 3,409,691, teaching removal of phenol by the use of macroporous ion exchange resin; U.S. Pat. No. 4,064,042, teaching separating an organic component such as phenol from blood by contact with a macroporous synthetic polymer; U.S. Pat. No. 5,218,132, teaching removal of aromatic impurities such as phenol by contact with a material including a smectite mineral; and JP 7215901A, teaching contact of a phenol-containing non-aqueous solvent with an acrylic weak basic ion exchange resin. See also U.S. Pat. Nos. 8,252,967; 7,022,161; 4,469,805; 4,404,118; 2,943,105; U.S. Patent Publication 2012-0316375 and U.S. patent application Ser. No. 13/618,211.
The present inventors have surprisingly discovered a method for selective removal of phenol from a process stream comprising hydrocarbon mixtures, especially hydrocarbon mixtures that contain higher-than-equilibrium paraxylene.